One example of the rotary electric machine in which permanent magnets are arrange inside of the rotor is a motor (a permanent magnet-type motor) having permanent magnets incorporated therein. As compared to an induction motor widely used in various fields, the permanent magnet-type motor is known as a highly efficient motor because an excitation current is unnecessary due to magnetic fluxes established by the permanent magnets incorporated in the rotor and no secondary copper loss occurs because no current flows in a rotor conductor unlike the induction motor. While the induction motor is conventionally used in railroad vehicles, application of a permanent magnet-type synchronous motor has been recently examined to achieve an efficiency enhancement, downscaling and an output increase, and simplification of a cooling structure.
The permanent magnet-type motor is roughly classified into a motor having a surface magnet structure (an SPM motor (Surface Permanent Magnet Motor)) in which permanent magnets are attached to a surface of a rotor, and a motor having an embedded magnet structure (an IPM motor (Interior Permanent Magnet Motor)) in which permanent magnets are embedded inside of a rotor. However, because a vehicle motor rotates at several thousand revolutions per minute, strength of the permanent magnets against centrifugal force at the time of high-speed rotation needs to be ensured. Accordingly, it can be said that the vehicle motor is almost limited to the IPM motor.
Conventionally, in an inverter for driving a vehicle motor, switching elements formed of silicon (Si) used as a base (hereinafter, “Si elements”) are generally used. Accordingly, a current (a motor current) flowing in the IPM motor needs to be reduced to enhance overall efficiency including the IPM motor and the inverter and an output voltage of the IPM motor needs to be increased to ensure a desired motor output. For example, in Patent Literature 1 mentioned below, a no-load induced voltage of an IPM motor is equal to or higher than an overhead wire voltage or overhead line voltage in the case of the maximum number of revolutions of the IPM motor.
Patent Literature 2 discloses a shape of an IPM motor in which magnets of a rotor are arranged in such a manner that two of the magnets for each pole form a V-shape and a heat transfer member for cooling is located at a central portion of each of the V-shapes.